1. Field of the Invention
The present invention relates to an image-taking apparatus, and more particularly to an image-taking apparatus that takes in an image of a subject optically through a zoom lens system and then outputs it in the form of an electrical signal by means of an image sensor, and among others to an image-taking apparatus provided with a compact, lightweight zoom lens system.
2. Description of the Prior Art
Today, digital cameras are quite popular. Digital cameras dispense with silver-halide film, and use instead an image sensor such as a CCD (charge-coupled device) or CMOS (complementary metal oxide semiconductor) sensor to convert an optical image into an electrical signal so that the optical image can be recorded and transferred in the form of digital data. A digital camera incorporates an image-taking apparatus provided with a lens system and an image sensor, and, in recent years, such image sensors have come to have increasingly large numbers of pixels. Correspondingly, the demand for high-performance image-taking apparatuses has been increasing greatly. Among others, compact image-taking apparatuses have been eagerly sought after that incorporate a zoom lens system that permits zooming without degrading image quality. In addition, in recent years, as the image processing performance of semiconductor devices and the like increases, more and more personal computers, mobile computers, cellular phones, personal digital assistants (PDAs), and the like have come to be internally or externally fitted with an image-taking apparatus having a zooming capability. This has been spurring on the demand for compact, high-performance image-taking apparatuses.
To make an image-taking apparatus compact, the zoom lens system incorporated therein needs to be made compact. Thus, many zoom lens systems have been proposed that aim at compactness. For example, three-unit zoom lens systems composed of three lens units, namely, from the object side thereof, a first lens unit having a negative optical power, a second lens unit having a positive optical power, and a third lens unit having a positive optical power are proposed in the following patent publications:                Publication 1: U.S. Pat. No. 6,532,114        Publication 2: Japanese Patent Application Laid-Open No. 2003-228002        Publication 3: Japanese Patent Application Laid-Open No. 2003-177314This type of zoom lens system, composed of a small number of optical elements and having a comparatively simple movement mechanism, has the advantage of being suitable to be made compact and other advantages.        
Publication 1 discloses zoom lens systems in which plastic lens elements are used as the most object-side lens element and as the most image-side lens element. Using plastic lenses makes introduction of aspherical surfaces easy, and helps reduce costs. However, with the constructions disclosed in Publication 1, it is difficult to achieve compactness while maintaining high optical performance. For example, in Examples 8 and 10 presented in Publication 1, plastic lens elements are used as the first and last lens elements. Here, however, the optical power of the first lens element is too weak to achieve satisfactory compactness. In the zoom lens system disclosed in Publication 2, a glass-molded lens element is used to introduce an aspherical surface. This, disadvantageously, increases costs. In the zoom lens system disclosed in Publication 3, although a plastic lens element is used to introduce an aspherical surface, the first lens unit is composed of a single lens element, and this makes correction of off-axial aberrations difficult.